Interface advance control in secondary recovery program by retarding cusp formation and reshaping of the interface between driving and driven fluids by the use of a dynamic gradient barrier

ABSTRACT

The interface between driving and driven fluids in a secondary recovery operation is retarded and/or reshaped after a cusp has developed by injection of fluids via wells controlling the advance of flow gradients and to delay the arrival of the injected driving fluid into the vicinity of a production well by the imposition of a dynamic gradient barrier of produced hydrocarbon fluids to spread the interface.

United States Patent I [72] Inventor Donald L. Hoyt Houston, Tex.

[21] Appl. No. 837,520

[22] Filed June 30, 1969 [45] Patented Sept. 28, 1971 [73] Assignee Texaco Inc.

New York, NY. Continuation-impart of application Ser. No. 786,565, Dec. 24, 1968, and a continuation-in-part of 786,568, Dec. 24, 1968.

[54] INTERFACE ADVANCE CONTROL IN SECONDARY RECOVERY PROGRAM BY RETARDING CUSP FORMATION AND RESHAPING OF THE INTERFACE BETWEEN DRIVING AND DRIVEN FLUIDS BY THE USE OF A DYNAMIC GRADIENT BARRIER 23 Claims, 25 Drawing Figs.

[52] U.S.Cl 166/245,

[51] Int. Cl ..E2lb43/20, E21b 43/22 [50] Field 01 Search 166/245, 263, 268, 266

[56] References Cited UNITED STATES PATENTS Re. 9/1960 Lindauer 166/268 24,873

3,074,481 1/1963 Haberrnann 166/245 3,109,487 11/1963 Hoyt 166/245 3,150,715 9/1964 Dietz 166/245 X 3,215,198 11/1965 Willman 166/263 Primary Examiner1an Av Calvert Att0rneys-l(. E. Kavanagh and Thomas H. Whaley ABSTRACT: The interface between driving and driven fluids in a secondary recovery operation is retarded and/or reshaped after a cusp has developed by injection of fluids via wells controlling the advance of flow gradients and to delay the arrival of the injected driving fluid into the vicinity of a production well by the imposition of a dynamic gradient barrier of produced hydrocarbon fluids to spread the interface.

PATENTEUSEPZBBII 3608635 sum 2 BF 5 I v j/ar/ phase 3 PATENTEU was 197: 1608;635-

PATENTED SEP28 can SHEET 5 [IF 5 INTERFACE ADVANCE CONTROL IN SECONDARY RECOVERY PROGRAM BY RETARDING CUSP FORMATION AND RESI-IAPING OF THE INTERFACE BETWEEN DRIVING AND DRIVEN FLUIDS BY THE USE OF A DYNAMIC GRADIENT BARRIER CROSS REFERENCES This application is a continuation-in-part application for patent of the copending, commonly assigned applications for U.S. Pat, Ser. No. 786,565, filed Dec. 24, 1968 by Donald L. Hoyt for Interface Advance Control in Secondary Recovery Program by Reshaping of the Interface Between Driving and Driven Fluids and Ser. No. 786,568, filed Dec. 24, 1968 by Donald L. Hoyt for Interface Advance Control in Secondary Recovery Program by Use of Gradient Barrier.

FIELD OF THE INVENTION This invention relates generally to the production of hydrocarbons from underground hydrocarbon-bearing formations, and more particularly, to a method for increasing the efficiency of the production of hydrocarbons therefrom by controlling the advance of the interface in pattern floods.

DESCRIPTION OF THE PRIOR ART In the production of hydrocarbons from permeable underground hydrocarbon-bearing formations, it is customary to drill one or more wells into the hydrocarbon-bearing formation and produce hydrocarbons, such as oil, through designated production wells, either by the natural formation pressure or by pumping the wells. Sooner or later, the flow of hydrocarbons diminishes and/or ceases, even though substantial quantities of hydrocarbons are still present in the underground formations.

Thus, secondary recovery programs are now an essential part of the overall planning for virtually every oil and gas-condensate reservoir in underground hydrocarbon-bearing formations. In general, this involves injecting an extraneous fluid, such as water or gas, into the reservoir zone to drive formation fluids including hydrocarbons toward production wells by the process frequently referred to as flooding. Usually, this flooding is accomplished by injecting through wells drilled in a geometric pattern, the most common pattern being the S-spot.

When driving fluid from the injection well reaches the production wells of a 5-spot pattern, the areal sweep is about 71 percent. By continuing production considerably past breakthrough, it is possible to produce much of the remaining unswept portion. It would be a great economic benefit to be able to achieve a sweep of mo percent of the hydrocarbonbearing formation. It would be an even greater benefit to be able to achieve it at breakthrough, so that it would not be necessary to produce large quantities of injected driving fluid.

It is understood that the failure of the driving flood in secondary recovery operations to contact or sweep all the hydrocarbon area is due to the development of a cusp at the interface between the driving and the driven fluids which advances toward the production well. If other portions of the interface could be made to keep up, or if the cusp formation were delayed, a more complete areal sweep would be possible.

In commonly assigned U.S. Pat. No. 3,393,735, issued to A.

F. Altamira et al. on July 23, 1968 for Interface Advance Control in Pattern Floods by Use of Control Wells, there is disclosed how an increased amount of hydrocarbons is produced and recovered from an underground hydrocarbon-bearing formation by employing at least three wells, penetrating such a fonnation, which wells are in line, to produce hydrocarbons from the formation via two of these wells including the middle well, as disclosed in the commonly assigned U.S. Pat. No. 3,109,487, issued to Donald L. l-loyt on Nov. 5, 1963 for Petroleum Production by Secondary Recovery. In both these cited patents, there is disclosed how a production control well is positioned between the injection well and the production well and is kept on production after the injected fluid reaches it. In this manner, the cusp is pinned" down at the control well and while the area swept out by the injection fluid before breakthrough at the outer production well is increased, there is an unwanted handling of considerable quantities of injected fluid at the control well.

Another aspect to increase the sweep is disclosed in the commonly assigned U.S. Pat. No. 3,393,734, issued to D. L. Hoyt et al. on July 23, 1968 and involves the retardation of the development of the cusp toward a production well. The method of achieving more uniform advance is to control the flow gradients so that the interface is spread" out. This can be done either by choosing a particular geometry of well positions or by adjusting the relative production rates so that the velocity of advance is not predominantly in one direction. It can be done also by shifting the gradients frequently, in both direction and magnitude, thus preventing any one section of an interface from advancing too far out of line.

SUMMARY OF INVENTION It is an overall object of the present invention to provide an improved recovery procedure involving initially at least two production wells substantially in line with a source of driving fluid (such as an injection well or active aquifer) and a fourth well offset from such a line, which wells may be part of a well arrangement for exploiting a hydrocarbon-bearing formation, by reshaping the interface between a driving fluid and formation fluids, after development of a cusp, into a configuration favorable to greater sweep and by changing the function of the wells at strategic times to gain maximum control of the flood front.

Such a four well grouping is arranged with three wells substantially in line and a fourth well offset therefrom so that an end well in line is completed for injection and the remaining three wells are completed for production. Flooding is initiated at the end well by injection thereinto of an extraneous driving fluid, such as water or gas, and proceeds until breakthrough of the flood front occurs at either the closer of the production wells in line or the production well offset therefrom, at which time injection via the end well to maintain flooding is suspended, the production wells may be put on a standby ba sis, and a small volume of an extraneous fluid is injected into the formation via the production well at which breakthrough occurred, to drive the cusp of the flood front therefrom and reshape the interface for the next production phase. Reshaping of the interface can be started at a predetermined time prior to breakthrough also. Then, injection is resumed at the end wall and production is continued or resumed at the other production wells, and the production well into which the extraneous fluid was injected is shut in.

The interface has now been reshaped so that instead of just one point, viz the tip of the cusp, being closest to the outer production well and thereby being accelerated to early breakthrough by the radial flow gradients, all the points over an appreciable portion the production the interface will now be at approximately equal travel time away from the outer production well, this travel time being greater than would have been the time for breakthrough without injection of the extraneous fluid via the well suffering breakthrough. In addition, the flanks of the flood front are reshaped, by being advanced during the period of injection of the extraneous fluid. Thus, more of the less accessible areas are swept, and the reversal of the cusp allows more sweep before breakthrough of the driving fluid into the outer production well. Examples of the extraneous fluid include produced formation hydrocarbon fluids, which may be treated with thickeners to increase the viscosity thereof, butane and propane, all being miscible with the formation fluids.

Upon continuing the injection of the extraneous fluid and the production of formation fluids, the flood front will cusp into either the other production well in line or the other of the original production wells, at which time injection of the extraneous fluid is suspended, the production wells are put on a standby basis, and a small volume of an extraneous fluid is injected into the formation via the production well at which breakthrough has just occurred. This injection of extraneous fluid will drive the cusp back and modify the interface for the next production phase. Then injection of extraneous driving fluid via the end well is resumed and injection of extraneous fluid via the converted well is continued and production is resumed.

The continuous injection into the converted well establishes a system of pressure gradients which on one side of the well are directed opposite to the pressure gradients associated with the driving fluid. A point of equilibrium of forces is established wherever the components of pressure gradient directed away from the converted well are equal and opposite to the components directed toward that well. The locus of all such equilibrium points establish a stable interface, normally teardrop shaped, around the converted well. The shape and size of this interface will depend upon the interrelationship of many factors, primarily, geometry of well positions, relative permeabilities and viscosities, and well rate distributions. Control of any of these factors can be used thereby to enhance the effectiveness of the method. Thus, with the offset well on production, the flow gradients are controlled so that the interface is spread out and the normal teardrop shape of the injected slug is changed toward the direction of the offset production well to result in a wider gradient barrier.

Since the injected driving fluid cannot penetrate this gradient barrier, it must travel a roundabout and longer flow path to reach the final production well, thereby delaying cusping into the end in line production well and allowing a longer period for the advance of the interface between the driving fluid and the formation fluids before breakthrough at this further well, When production at this end well is continued after breakthrough, the converted well is shut-in and the continuing production results in recovery of the injected produced hydrocarbon fluids, along with remaining in place formation fluids.

Many variations of this basic procedure are possible, and some will be more advantageous than others for particular geometries of well positions, and reservoir and fluid parameters. But they will have certain things in common:

1. An intermediate well between and a well offset from the line through an injection source and a production well either exist or are added;

2. At breakthrough into either this intermediate well or the offset well (or some time prior thereto), the injection of driving fluid is suspended and a volume of fluid, such as a portion of the produced hydrocarbon fluids, is injected into the well suffering breakthrough; this injection may be done with or without simultaneous production from other wells;

3. Injection of driving fluid is resumed at the initial injection well, and injection of the barrier fluid is continued into the well suffering breakthrough at a rate equal to a percentage of the production rates;

4. This continues, even though other existing wells may be captured by injected driving fluid and closed in, until breakthrough of the injection fluid into the production well on the other side of the converted well;

5. At this time, or in some cases even before it, the injection of barrier fluid is stopped; continued injection of driving fluid on one side and continued production on the other will move the barrier fluid completely into the production well if recovery of this fluid is desired (as, for example, if the barrier fluid used comprises produced hydrocarbon fluids).

Other objects, advantages and features of this invention will become apparent from a consideration of the specification with reference to the figures of the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 discloses four units of an inverted 5-spot pattern (Prior Art);

FIG. 1a is illustrative of the interface advance in the form of a cusp toward a corner production well in one quadrant of such a S-spot pattern undergoing secondary recover (Prior FIG. 2 illustrates the movement of the interface during phases of a production program in one quadrant of a 13-well quadrilateral side well pattern undergoing secondary recovery (Prior Art);

FIG. 3a discloses one quadrant of a nine-well diagonal pattern, illustrating the breakthrough at a comer production well, P following the reshaping of the cusp at the control well between the injection well and the comer production well resulting from injection of a small volume of produced hydrocarbon fluids into a control well, with all other wells temporarily shut-in;

FIG. 3b discloses one quadrant of a nine-well diagonal pattern, illustrating the breakthrough at a comer production well, P following the retreat of the cusp at the control well between the injection well and the comer production well resulting from injection of a small volume of produced hydrocarbon fluids into a control well, with all other wells temporarily shut-in,

FIGS. 40 and 4b and FIGS. 5a and 5b illustrate the effects of an offset production well on a line drive of at least three wells wherein the cusp interface is reshaped at the offset well and a dynamic barrier is imposed on the in line production well in turn as each suffers breakthrough, and vice versa;

FIGS. 6a and 6b, FIGS. 70 and 7b and FIGS. 8a and 8b illustrate alternate showings of the effect of an offset production well in a five-well grouping, spreading the cusp interface and with cusp reshaping and/or dynamic barriers imposed on it and the in line production well in turn, as they suffer breakthrough,

FIGS. 9a, 9b and 9c and FIGS. 10a, 10b and 10c illustrate the application of the principles of FIGS. 4b and 5b to a uniformly spaced seven-well grouping, starting at one end of the long diagonal thereof; and

FIGS. 11a, 11b, 11c and 11d illustrate the changes in well functions in accordance with the movement of the spread interface during the several phases of the production program in a 13-well grouping undergoing secondary recovery.

The objects of the invention are achieved by the use of a combination of production wells used as control wells to delay the breakthrough or modify the interface of injected driving fluid into the outermost production wells, thereby containing more sweep and recovery of formation hydrocarbons before breakthrough of the driving fluid.

The specification and the figures of the drawings schematically disclose and illustrate the practice and the advantages of the invention with well patterns and areal sweep examples which are obtainable and have been observed both in secondary recovery operations and in potentiometric model studies which simulate secondary recovery operations. The model studies indicate a sweep obtained in an ideal reservoir, although the recovery from an actual sweep of a particular field may be greater or less, depending on field parameters.

Throughout the figures of the drawings, the same symbols will be maintained as follows: P P, and Pi represent respectively production wells at the comers, along the sides, and the interior control wells of pattern or well arrangement; the left diagonal represents the reinjected produced hydrocarbon fluids; and, a solid circle indicates a production well, a crossed circle indicates a shut in well, an open circle a well site, an up wardly arrowed open circle indicates an injection well, and a downwardly arrowed solid circle, a converted well. The diagonal x-x in FIGS. 11a, 11b, 11c and 11d represents an axis of an injection well and a pair of in line production wells.

Referring to FIG. 1, there is disclosed four units of an inverted S-spot pattern wherein the comer wells of each pattern unit are production wells, while the inner central well is used for injection.

FIG. 1a illustrates the growth of the cusp in one quadrant of an inverted S-spot pattern unit, wherein the secondary flooding fluid is injected into the central well and production is maintained at the corner wells until breakthrough, to result in a sweep approximately 71 percent.

FIG. 2 illustrates the development of the cusp toward the comer production well, retarded by spreading of the gradients, production being maintained at both the corner and side production wells until breakthrough at the side wells at which time they are shut in for the end of the first phase, and then continuing production at the corner wells only, until breakthrough, for the end of the second phase.

Referring to FIG. 3a, there is disclosed a ninewell diagonal pattern, essentially the S-spot pattern with control wells positioned on the diagonals between the central injection well and the comer production wells. The control wells should be spaced at least one-half the distance between the injection well and each corner production well, with the best results obtained when such control wells are positioned between threequarters and seven-eights of the distance from the injection well toward the comer production wells. With such a pattern, the invention disclosed in the cited patent to Hoyt can be employed with success to increase the sweep area over that mentioned for the basic S-spot pattern.

Use of control wells as disclosed by above-cited patents to Hoyt and to Altamira et al., by continuing production from P, after breakthrough even to l00 percent injected fluid, will have the beneficial effect but at the cost and difficulty of handling and disposing of large quantities of the driving fluid.

Instead, if a small volume, e.g. about percent of a quadrant of a pattern unit volume of produced formation hydrocarbon fluids, is injected into the formation via the control well, P,, from which production has ceased, the cusp will be driven back, and flank portions of the interface or flood front will be advanced (as indicated by the line A,-A in FIG. 3a), by the injected bubble of hydrocarbon fluids, shown in section with left diagonals in outline D,.

When production is initiated at the corner well, P with P, shut-in, and injection resumed at the central well, the interface and bubble both move toward the production well, P Now, however, all the points between a and b on the interface are approximately the same travel time from P and as the flood front progresses, the bubble is reproduced and the points between a-b converge into the production well, yielding at breakthrough, a sweep of 87 percent, as outlined at B P B The advantage of high sweep is thereby realized without handling injection fluid.

If satisfactory production of reservoir fluid along with injection fluid is possible in a given reservoir, (it often is not) this percentage can be increased by continued production.

The improvement in sweep can be optimized in several ways. The size of the bubble, the geometry of the wells, the relative rate distribution and judicious use of the central injection well during the reshaping phase, can be used in any given operation to mold the interface into the best shape for the next production phase.

In FIG. 3b, the invention illustrates the improvement provided by one embodiment of this invention over the disclosures of the prior art in FIGS. 1, 1a, and 2. With injection of the driving fluid via the central well of a nine-well diagonal grouping, production is maintained via the interior'control well, P,, (or also via the comer production well, P until breakthrough thereat, to form the interface indicated at A,P,A in FIG. 3b, to yield a sweep of 57 percent. The control well, P is located on the diagonal through the injection well and the comer production well, P about three-quarters of the distance from the central injection well. Ifthe production well at P, were closed in to avoid handling any of the injected driving fluid and production were initiated (or continued) and maintained till breakthrough at the comer well, P the sweep (not indicated) would increase by 9 percent, for a total sweep of 66 percent.

Instead, a volume of produced formation hydrocarbon fluids equal to a predetermined value, e.g. about 15 percent of a pattern unit volume, is injected through the internal control well, P,, from which production has ceased, and the cusp driven back by the resulting injected bubble of hydrocarbon fluids, as shown in section with left diagonals at C FIG. 3b, and the interface distorted as indicated by the dash outline at A A the point of the cusp being driven back from the control well, P while the flanks advance from the line A P A The changes in the shapes of the sides of the interface have been exaggerated for purposes of clarity.

When production is initiated (or resumed) at the corner well, P and with about 50 percent of produced hydrocarbon fluids being injected continuously, into the formation via well P,, the cycling hydrocarbon fluids form a stable teardrop bubble, as indicated in left diagonal sectioning at C FIG. 3b. The envelope of this bubble represents the surface of equilibrium of pressure gradient forces.

By the time the driving fluid gets around the gradient barrier to achieve breakthrough at the corner production well, P the sweep of the formation fluids has been increased 31 percent, as indicated by the interface at BgP BZ, for a total sweep of 88 percent.

Finally, if production at P is continued after breakthrough with the control well P, closed in, maximum gradients are reestablished along the axis of the wells, and virtually all the injected produced fluid hydrocarbon fluids can be recovered quickly along with additional formation fluids miscible therewith, bringing total sweep to over 90 percent before an appreciable percentage of injected driving fluid id produced.

The sweep can be increased either by forming a larger initial bubble or using a greater fraction of the produced hydrocarbon fluids for injection back into the formation via the converted control well.

FIG. 4a illustrates phases of an in line production drive wherein a predetermined amount of produced hydrocarbon fluids is injected (as the second phase to reshape the cusp) into the formation via the offset well 4, at which breakthrough of the driving fluid, injected as the first phase via the first well in line, 1, has occurred. During the third phase following the reinjection via well 4, the second and third wells in line, 2 and 3, are placed on production to spread the cusp, and .the fourth well, 4, is shut-in, the start of the third phase being illustrated in FIG. 4a, with injection at 1, with sweep indicated by right diagonals and reinjected fluids at 4 by left diagonals and production at 2 and 3, until breakthrough at the former, as indicated by the dash outline in FIG. 4b, showing the spreading of the cusp interface. Upon breakthrough of the interface at well 2, this production well is converted into an injection well for produced hydrocarbon fluids as the fourth phase, with in jection of extraneous fluids via well 1 being continued, well 4 being shut-in and production maintained from well 3. The end of the fifth phase concludes at breakthrough at 3, leaving only a lens of produced hydrocarbon fluids, wells 2 and 4 having been shut-in and injection being continued via well 1.

In FIG. 5a, production via wells 2 and 4 (and well 3 if so programmed) continues as the first phase till breakthrough of the driving fluid injected via well 1, at which time, a slug of produced hydrocarbon fluids is injected back into the formation as the second phase via the well suffering breakthrough to reshape the interface, which may be either the production well in line 2, as illustrated, or the offset well 4, toward which the cusp has spread. Thereupon, production is either initiated at well 3 or resumed at wells 3 and 4, with injection via well 1 until breakthrough at well 4, the start of this phase being illustrated in FIG. 5a.

In F IG, 5b, the ends of the third, fourth, and fifth phases are illustrated with breakthrough at the third well in line, 3, well 2 being shut-in, a dynamic barrier of produced hydrocarbon fluids being imposed on well 4 as the fourth phase and injection being maintained via well 1. The dot-dash outline ending at 3 indicates how much the cusp has spread.

FIGS. 6a and 6b show the application of this invention to a five well grouping. In the first figure, the end of the first phase of production occurs with breakthrough at the offset wells 4 and 4', the in line production wells being on standby status and injection occurring via well 1. The second phase involves the reshaping of the cusp interface at the breakthrough at offset wells 4 and 4' by the injection of produced hydrocarbon fluids thereinto, and the third phase occurs when the cusp is spread till breakthrough at 2, by production therefrom and from well 3, ofiset wells 4 and 4 having been shut-in.

The ends of the next two phases are shown in FIG. 6b, with a dynamic barrier of produced hydrocarbon fluids being imposed on well 2 and injection of driving fluid via well 1, as the fourth phase, and then ending the fifth phase with production till breakthrough at 3.

FIGS. 70 and 7b also illustrate the application of this invention to a five well grouping. In the first figure, the end of the first phase of production occurs with breakthrough at the offset wells 4 and 4', the in line production wells being on standby status and injection occurring via well 1. The second phase involves the imposition of a dynamic barrier of produced hydrocarbon fluids via wells 4 and 4' and spreading the cusp by production till breakthrough at 2 and from well 3.

The ends of the next phases are shown in FIG. 7b with a reshaping of the cusp at well 2, by injecting produced hydrocarbon fluids thereat and then shutting in this well for the end of the third phase, the dynamic barriers being maintained at the offset wells 4 and 4', with the end of the fourth phase shown by dot-dash outline with production till breakthrough at 3.

FIG. 8a illustrates an alternate embodiment of the invention as applied to a five well grouping. Upon breakthrough of the' driving fluid injected via well 1 as the end of the first phase, a dynamic barrier of produced hydrocarbon fluids is imposed at well 2 as the second phase and the cusp interface is spread by production from the offset wells 4 and 4 till breakthrough thereat. Thereupon, the cusp interface is reshaped at these wells by the injection of produced hydrocarbon fluids as the third phase and production is completed at the fourth phase upon breakthrough at the in line well 3.

FIG. 8b is the converse of the illustration of FIG. 8a, reshaping the cusp interface at in line production well 2, as the end of the second phase and imposing dynamic barriers at offset production wells 4 and 4', following breakthroughs thereat, shown as the end of the third phase. Injection is via well 1 and production ceases upon breakthrough at in line production well 3.

FIGS. 9:: to 90 and FIGS. 10a to 100, both inclusive, illustrate how the disclosed invention may be applied to a geometrically spaced 7 well grouping. In FIGS. 9a, 9b and 9c, the production operation is initiated from offset side wells 4 and 4' till breakthrough and reshaping of the cusp interface thereat, then continued from in line well 2 till breakthrough thereat, followed by imposition of a dynamic barrier, FIG. 9b, then spreading the cusp by continuing production from the other offset side wells and S till breakthrough and reshaping the cusps thereat, and then producing to conclusion at the remaining in line well 3.

The converse of this operation is illustrated in FIGS. a, 10b and 10c.

Upon breakthrough of the driving fluid injected via well 1 at the offset wells 4 and 4, dynamic barriers are imposed thereupon and production continued from well 2 till breakthrough thereat, as illustrated in FIG. 100, the cusp interface having been spread by the offset well production and then by the in line production.

In FIG. 10b, production is continued via the offset wells 5 and 5' (and after reshaping of the cusp at in line well 2) to spread the interface between the driving and driven fluids.

Upon breakthrough at the latter production offset side wells 5 and 5', dynamic barriers are imposed thereupon and production is continued till breakthrough via the remaining in line well 3, as indicated in FIG. 10c.

FIGS. 11a, 11b, 11c and 11d disclose the basic 9-spot pattern modified by the addition of four interior control wells, which can be positioned along the diagonals of the pattern for best advantage as indicated previously. It can be visualized also as a four unit 5-spot pattern, wherein the injection wells of the inverted 5-spot pattern units have been converted to production wells, and the innermost production well of the four unit 5-spot pattern has been converted into an injection well. With such a conversion, the positions of the control wells have been predetermined and may not be situated for best effeet.

As illustrated in one quadrant of the pattern, the first phase of the production method requires injecting driving fluid via the central well and production initiated and maintained at the remaining 12 wells of the pattern until breakthrough is achieved at the four interior control wells, as shown by the dash outline in FIG. 11a. Then these interior production wells are converted to injection wells for receiving produced formation hydrocarbon fluids to provide a dynamic barrier while production is continued from the side wells P, spreading the cusp, until breakthrough thereat, as illustrated by the solid outline in FIG. 11a, the left diagonal cross section D indicating returned produced hydrocarbon fluids, the corner production wells P being in a standby status. As indicated in FIG. 11b, the four side wells are converted to injection wells for produced hydrocarbon fluids to reshape the cusp interface thereat and then shut-in, the bubble being shown in dash outline, and production is resumed thereafter at the corner production wells P until breakthrough of injection fluid occurs thereat. If production after breakthrough at the corner wells is continued, it becomes feasible to recover the teardrop bubbles for additional sweep, leaving slivers of unswept areas adjacent the comer production wells.

Alternatively, as illustrated in FIGS. 11c and 11b for one quadrant of a 13 well grouping, the first phase of the production method requires injection driving fluid via the central well and production initiated and maintained at the interior and side wells of the pattern until breakthrough is achieved at the four interior wells, as shown by the dash outline in FIG. 110. Then these interior production wells are converted to injection wells for receiving produced formation fluid hydrocarbons (bubble D to reshape the cusp interface and then shutin, after which production is continued from the side wells P, until breakthrough thereat, as illustrated, the left diagonals in this and following figure indicating the returned produced hydrocarbon fluids. Then, as indicated in FIG. 11d, the four side wells are converted to injection wells to form a dynamic barrier of produced hydrocarbon fluids thereat, shown in outline, production is initiated and maintained at the corner production wells P until the surface has progressed through the stages indicated in this figure, viz the bubble D being reproduced, the interface surface a-c-b converges and breakthrough occurs.

In any kind of a secondary recovery operation in which one fluid is moved by another toward what is essentially a point sink, such as a production well, the radial flow gradients in the vicinity of the production well inevitably cause the interface to form a cusp pointing toward that well. The various fluid and field parameters, such as permeability distribution, viscosity ratio, well geometry, types of drives, miscibility, displacement efficiencies, etc., will cause the cusp to form earlier or later and be more or less pronounced, but it will form always, and always it will be detrimental to efficient sweep of any region being considered.

The ideal sweep situation for a reservoir would be one in which the fluid interface, as it approaches the last production well, somehow would be in such a geometrical position that all points on it would require the same time to reach the well. The method of spreading the cusp disclosed in U.S. Pat. No. 3,393,734 (to I-Ioyt et al.) is successful because it approaches this ideal.

There will be times, however, when the cusp cannot be spread" out, and the presently disclosed method offers a convenient and workable alternative. All that is required is that there be at least two wells substantially in line with a source of driving fluid such as an injection well or an active aquifer. The principle involved is as follows:

The point of the cusp forms along the line of strongest gradient between the injection source and the nearest production well. If fluid is injected into the production well, which is here called the control well, the system is reversed and the point of the cusp is driven back and away from the next production well in line, by the fluid injected via the control well. The interface between the control well fluid and the injection well fluid becomes concave. If the proper size of bubble is introduced, to fit the particular well geometry, the concavity may be sized such that all or most of the points on it (between a" and b," e.g. in FIG. 3a) will be in gradient fields such that their time of travel to the production well will be equal.

Resumption of injection of driving fluid and of production from the well beyond the control well will yield a very high sweep. The wider the distance between points a and b," the greater, in general, will be the sweep at breakthrough of these points into the producing well. This distance can be increased if the points of injection should straddle the axis extending through the injection and production wells.

Reservoir hydrocarbons are particularly suitable here for control well injection fluid, since they will follow most closely model study predictions, and will be recovered without difficulty in the subsequent production phase. However, any fluid of properties (particularly viscosity and miscibility) similar to the reservoir fluid can be used effecn'vely.

The effect of continuous injection of a control well fluid into the formation between the well injecting driving fluid and a production well is to create an interior envelope of cycling fluid between the control well and the production well which the driving fluid cannot penetrate. This condition forces the driving fluid to move around the barrier thus retarding cusp formation toward the production well, giving longer time for sweep before interface breakthrough. Clearly, the wider the barrier can be made, the better will be the sweep.

Factors which will cause such a barrier to be wide are:

A. higher viscosity of the control well fluid than of the driving fluid;

B. high control well injection rates as a percentage of production;

C. production from side wells during the control well injection phase;

D. dual control wells straddling the axis through the injection and production wells to from a wider bubble, depending on the spacing between the straddle wells.

Any pattern and/or rate distribution which retards the development, or the advance, of a cusp towards production wells will increase the sweep of a field.

Herein has been disclosed another method of delaying the advance of the interface in the form of a cusp toward a production well by locating a dynamic barrier of produced hydrocarbon fluids between an injection well and the outer production well and/or reshaping the cusp interface, either prior to or at breakthrough, in combination with a spreading of the cusp by the use of offset side wells in an in line production drive.

Although emphasis has been placed in this disclosure on the practice of this invention as directed to a secondary recovery operation, particularly employing water or other similar aqueous fluid as the injection displacement fluid, the advantages obtainable in the practice of this invention are also realized in primary hydrocarbon production operations wherein the hydrocarbon-bearing formation is under the influence of either a water or gas drive, or both a water and gas drive, and also in the instance of a secondary recovery operation wherein a gas, such as natural gas, is employed as the injection fluid. Moreover, the invention is applicable particularly to an arrangement of a pair of production wells in line and a production well offset therefrom with an injection well under the influence of an active water drive.

Iclaim:

1. A method of producing formation fluids including hydrocarbons from an underground hydrocarbon-bearing formation which comprises penetrating said formation with at least three wells substantially in line, a first well, a second well and a third well, the second and third wells being on one side of said first well with said second well closer thereto and a well offset from the line of wells and adjacent said second well, injecting an extraneous fluid into said formation via said first well to displace fluids including hydrocarbons in said formation toward said second and third wells and the offset well, producing said fonnation fluids including hydrocarbons from said fonnation via the second and offset wells, recovering formation hydrocarbon fluids from produced formation fluids, ceasing producing said formation fluids via the production well suffering breakthrough of said extraneous fluid thereat and thereupon injecting a portion of said formation hydrocarbon fluids into said formation via said production well suffering breakthrough while injecting extraneous fluid via said first well and thereafter producing formation fluids via said third well and the other original production well until breakthrough of said extraneous fluid at one thereof, thereupon ceasing injecting said extraneous fluid and said formation hydrocarbon fluids and ceasing production formation fluid and injecting into the last-mentioned production well suffering breakthrough a portion of formation hydrocarbon fluids and then shutting in such well and resuming injecting the extraneous and hydrocarbon fluids and producing formation fluids from the remaining production well.

2. In a method as defined in claim 1, maintaining producing said formation fluids including hydrocarbons from said formation via said third well and said other original production well while maintaining injecting formation hydrocarbon fluids into said formation via said producing well suffering breakthrough to provide a dynamic gradient barrier.

3. In a method as defined in claim 1, the production of formation fluids via said second and offset wells being simultaneous.

4. In a method as defined in claim 1, continuing injecting produced formation hydrocarbon fluids via said well suffering breakthrough first for a predetermined period while maintaining producing via the remaining production wells.

5. In a method as defined in claim 1, said injecting of hydrocarbon fluids via said production well suffering breakthrough first being of predetermined percentage of the pattern unit volume in the amount of about 15 percent, said extraneous fluid being selected from the group consisting of butane, propane and produced hydrocarbon fluids.

6. In a method of producing formation fluids including hydrocarbons as defined in claim 1, said offset well being one of a pair which straddles the line of three wells.

7. In a method of producing formation fluids as defined in claim 6, said three wells in line being part of a five well groupmg.

8. In a method of producing formation fluids as defined in claim 6, said three wells in line being part of a seven well geometric grouping.

9. In a method for producing formation fluids as defined in claim 6, said three wells in line being part of a 13 well grouping, wherein the central well of said grouping is an injection well and the remaining grouping wells are production wells arranged equally along the sides and on the diagonals of a quadrilateral, said second and third wells being arranged along a diagonal thereof.

10. In a method of producing formation fluids as defined in claim 9, simultaneously initiating producing said formation fluids via all of said production wells.

11. In a method of producing formation as defined in claim 9, producing formation fluids via said wells located on the diagonals of said pattern adjacent said injection well and continuing producing therefrom until breakthrough of said extraneous fluid occurs, thereupon converting said aforementioned diagonal wells into injection wells and injecting into said formation via such converted wells produced formation hydrocarbon fluids, in the amount of 15 percent of the pattern unit volume, and producing from the side wells of said pattern until extraneous fluid breakthrough occurs thereat, thereupon ceasing injection extraneous fluid and converting said side wells into injection wells for injecting a portion of produced formation hydrocarbon fluids and then shutting in such wells and resuming injecting extraneous fluid into said formation via said central well and producing said formation fluids via the comer wells of said pattern until extraneous fluid breakthrough occurs thereat.

12. In a method as defined in claim 1 1, producing fonnation fluids from said side wells and said comer wells being concurrent.

13. In a method as defined in claim 11, producing formation fluids from said side wells and said corner wells being in turn.

14. A method of producing formation fluids including hydrocarbons from an underground hydrocarbon-bearing formation which comprises penetrating said formation with at least three wells substantially in line, a first well, a second well and a third well, the second and third wells being on one side of said first well with said second well closer thereto and a well offset from the line of wells and adjacent said second well, injecting an extraneous fluid into said formation via said first well to displace fluids including hydrocarbons in said formation toward said second and third wells and the offset well, producing said formation fluids including hydrocarbons from said formation via the second and offset wells, recovering formation hydrocarbon fluids from produced formation fluids, ceasing injecting said extraneous fluid and ceasing producing said formation fluids upon breakthrough of said extraneous fluid at a production well and then injecting an extraneous fluid into said formation via said last-mentioned well and then shutting in such well, and producing said formation fluids including hydrocarbons from said formation via said third well and the other original production well while injecting extraneous fluid into said formation via said first well until breakthrough of said extraneous fluid at one production well thereof, thereupon ceasing producing at such well suffering breakthrough secondly and injecting a portion of formation fluids thereinto while injecting extraneous fluid and producing formation fluids via the remaining wells.

15. In a method as defined in claim 14, said injecting of hydrocarbon fluids via the second well suffering breakthrough being of predetermined volume, starting with a percentage of the pattern unit volume and continuing with a percentage of the produced formation fluids, while producing said formation fluids from said remaining wells, to provide a dynamic gradient barrier therebetween.

16. In a method as defined in claim 14, said extraneous fluid injected via said first well and said production well suflering breakthrough being selected from the group consisting of butane, propane and produced hydrocarbon fluids.

17. In a method of producing formation fluids as defined in claim 14, said three wells in line being part of a thirteen well pattern, wherein the central well of said pattern is an injection well and the remaining pattern wells are production wells arranged in equal numbers along the sides and on the diagonals of a quadrilateral.

18. In a method of producing formation fluids as defined in claim 17, simultaneously initiating producing said formation fluids via all of said production wells.

19. In a method of producing formation fluids including hydrocarbons as defined in claim 14, said offset well being one of a pair which straddles the line of three wells.

20. A method of producing formation fluids including hydrocarbons from an underground hydrocarbon-bearing formation under the influence of an active aquifer which comprises penetrating said formation with a pair of production wells in line with the direction of advance of said aquifer, and a third production well offset from the line of production wells and adjacent one of said pair of production wells, producing formation fluids including hydrocarbons displaced by said aquifer via said pair of production wells and the offset well; until breakthrough of the interface between the formation fluids and said aquifer at a production well, thereupon ceasing producing formation fluids thereat and injecting thereinto an extraneous fluid of predetermined volume and then shutting in this well, thereupon producing formation fluids including hydrocarbons from said formation via the remainin production wells until breakthrough of extraneous flur at one thereof, then converting such well to an injection well for an extraneous fluid to provide a dynamic gradient barrier at the interface where said breakthrough has occurred while producing formation fluids via the remaining production well.

21. In a method of producing formation fluids including hydrocarbons as defined in claim 20, said extraneous fluid being selected from the group consisting of butane, propane and produced hydrocarbon fluids.

22. In a method of producing formation fluids including hydrocarbons as defined in claim 21, said predetermined volume amounting to about 15 percent of the pattern unit volume.

23. In a method of producing fonnation fluids including hydrocarbons as defined in claim 20, said offset well being one of a pair which straddles the line of production wells. 

2. In a method as defined in claim 1, maintaining producing said formation fluids including hydrocarbons from said formation via said third well and said other original production well while maintaining injecting formation hydrocarbon fluids into said formation via said production well suffering breakthrough to provide a dynamic gradient barrier.
 3. In a method as defined in claim 1, the production of formation fluids via said second and offset wells being simultaneous.
 4. In a method as defined in claim 1, continuing injecting produced formation hydrocarbon fluids via said well suffering breakthrough first for a predetermined period while maintaining producing via the remaining production wells.
 5. In a method as defined in claim 1, said injecting of hydrocarbon fluids via said production well suffering breakthrough first being of predetermined percentage of the pattern unit volume in the amount of about 15 percent, said extraneous fluid being selected from the group consisting of butane, propane and produced hydrocarbon fluids.
 6. In a method of producing formation fluids including hydrocarbons as defined in claim 1, said offset well being one of a pair which straddles the line of three wells.
 7. In a method of producing formation fluids as defined in claim 6, said three wells in line being part of a five well grouping.
 8. In a method of producing formation fluids as defined in claim 6, said three wells in line being part of a seven well geometric grouping.
 9. In a method of producing formation fluids as defined in claim 6, said three wells in line being part of a 13 well grouping, wherein the central well of said grouping is an injection well and the remaining grouping wells are production wells arranged equally along the sides and on the diagonals of a quadrilateral, said second and third wells being arranged along a diagonal thereof.
 10. In a method of producing formation fluids as defined in claim 9, simultaneously initiating producing said formation fluids via all of said production wells.
 11. In a method of producing formation fluids as defined in claim 9, producing formation fluids via said wells located on the diagonals of said pattern adjacent said injection well and continuing producing therefrom until breakthrough of said extraneous fluid occurs, thereupon converting said aforementioned diagonal wells into injection wells and injecting into said formation via such converted wells produced formation hydrocarbon fluids, in the amount of 15 percent of the pattern unit volume, and producing from the side wells of said pattern until extraneous fluid breakthrough occurs thereat, thereupon ceasing injection extraneous fluid and converting said side wells into injection wells for injecting a Portion of produced formation hydrocarbon fluids and then shutting in such wells and resuming injecting extraneous fluid into said formation via said central well and producing said formation fluids via the corner wells of said pattern until extraneous fluid breakthrough occurs thereat.
 12. In a method as defined in claim 11, producing formation fluids from said side wells and said corner wells being concurrent.
 13. In a method as defined in claim 11, producing formation fluids from said side wells and said corner wells being in turn.
 14. A method of producing formation fluids including hydrocarbons from an underground hydrocarbon-bearing formation which comprises penetrating said formation with at least three wells substantially in line, a first well, a second well and a third well, the second and third wells being on one side of said first well with said second well closer thereto and a well offset from the line of wells and adjacent said second well, injecting an extraneous fluid into said formation via said first well to displace fluids including hydrocarbons in said formation toward said second and third wells and the offset well, producing said formation fluids including hydrocarbons from said formation via the second and offset wells, recovering formation hydrocarbon fluids from produced formation fluids, ceasing injecting said extraneous fluid and ceasing producing said formation fluids upon breakthrough of said extraneous fluid at a production well and then injecting an extraneous fluid into said formation via said last-mentioned well and then shutting in such well, and producing said formation fluids including hydrocarbons from said formation via said third well and the other original production well while injecting extraneous fluid into said formation via said first well until breakthrough of said extraneous fluid at one production well thereof, thereupon ceasing producing at such well suffering breakthrough secondly and injecting a portion of formation fluids thereinto while injecting extraneous fluid and producing formation fluids via the remaining wells.
 15. In a method as defined in claim 14, said injecting of hydrocarbon fluids via the second well suffering breakthrough being of predetermined volume, starting with a percentage of the pattern unit volume and continuing with a percentage of the produced formation fluids, while producing said formation fluids from said remaining wells, to provide a dynamic gradient barrier therebetween.
 16. In a method as defined in claim 14, said extraneous fluid injected via said first well and said production well suffering breakthrough being selected from the group consisting of butane, propane and produced hydrocarbon fluids.
 17. In a method of producing formation fluids as defined in claim 14, said three wells in line being part of a thirteen well pattern, wherein the central well of said pattern is an injection well and the remaining pattern wells are production wells arranged in equal numbers along the sides and on the diagonals of a quadrilateral.
 18. In a method of producing formation fluids as defined in claim 17, simultaneously initiating producing said formation fluids via all of said production wells.
 19. In a method of producing formation fluids including hydrocarbons as defined in claim 14, said offset well being one of a pair which straddles the line of three wells.
 20. A method of producing formation fluids including hydrocarbons from an underground hydrocarbon-bearing formation under the influence of an active aquifer which comprises penetrating said formation with a pair of production wells in line with the direction of advance of said aquifer, and a third production well offset from the line of production wells and adjacent one of said pair of production wells, producing formation fluids including hydrocarbons displaced by said aquifer via said pair of production wells and the offset well; until breakthrough of the interface between the formation fluids and said aquifer at a produCtion well, thereupon ceasing producing formation fluids thereat and injecting thereinto an extraneous fluid of predetermined volume and then shutting in this well, thereupon producing formation fluids including hydrocarbons from said formation via the remaining production wells until breakthrough of extraneous fluid at one thereof, then converting such well to an injection well for an extraneous fluid to provide a dynamic gradient barrier at the interface where said breakthrough has occurred while producing formation fluids via the remaining production well.
 21. In a method of producing formation fluids including hydrocarbons as defined in claim 20, said extraneous fluid being selected from the group consisting of butane, propane and produced hydrocarbon fluids.
 22. In a method of producing formation fluids including hydrocarbons as defined in claim 21, said predetermined volume amounting to about 15 percent of the pattern unit volume.
 23. In a method of producing formation fluids including hydrocarbons as defined in claim 20, said offset well being one of a pair which straddles the line of production wells. 